Add MP3 files to my music library

How to add MP3 files to my music library. I have the MP3 files already in the "Files" area on my iPhone (and on my PC). How do I place these MP3 files into my music library so I can play them my iPhone or in my car through Apple play? These are MP3 music files I created, not purchased or downloaded.

Revisions: ios 14.0.1, iTunes 12.10.9.3